F-Doped NaTi₂(PO₄)₃/C Nanocomposite as a High-Performance Anode for Sodium-Ion Batteries

We are presenting a sol-gel method for building novel nanostructures made of nanosized F-doped Na_{1-2 x}Ti₂(PO₄)_{3- x}F _x (NTP-F _x, x = 0, 0.02, 0.05, and 0.10) particles embedded in three-dimensional (3D) carbon matrices (NTP-F _x/C). This technique combines advantages of both zero-dimensional materials and 3D-carbon networks. Proper fluorine doping stabilizes the NTP structure and greatly enhances ion/electron transportation, leading to superhigh-rate electrochemical performance and ultralong cycle life. The composite electrode delivers high specific capacities of 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mA h g^-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. It retains an unbelievable ∼70% capacity after a thousand cycles at a rate as high as 10 C. Electroanalytical results reveal that fluorine doping significantly enhances Na⁺ diffusion kinetics. Meanwhile, density functional theory calculations demonstrate F-doped NTPs' own outstanding electrochemical properties, which is due to the enhanced intrinsic ionic/electronic conductivity. The results show that anion doping is an efficient way to make high-performance NTP anodes for sodium-ion batteries.